1. Background of the Invention
The invention relates to a method of bending a laminate having a thermo-plastic core layer and metal sheets adjoining the core layer on both sides, wherein the laminate is heated and then bent while hot.
2. Description of the Prior Art
EP-A-0013146 describes numerous laminates which have a plastic core layer and metal skin layers on both sides. A problem with these known laminates is that their deformability is moderate and that even a simple bending operation for making an angle of 90.degree. with a bending radius equal to two times the thickness of the laminate is often not possible without causing damage to the laminate. EP-A-0013146 mentions, as a possible solution, the heating of the laminate, but is has been found that in many cases this only leads to a slight improvement in deformability.
Especially when the laminate is given a bend with a bending radius of the same order as thickness of the laminate, it is generally not possible to do so without cracking the outer metal skin. Such small bending radii are used, for example, in laminates for car body parts, wherein an edge suitable for seaming has to be made. In such a case the laminate may comprise, for example, a core layer of acrylonitrile-butadiene-styrene (ABS), polyethylene terephthalate (PET) or polypropylene (PP), and for the skin layers steel or aluminium may be used. Examples of such laminates are described in co-pending application Ser. No. 07/759,915, filed Sep. 13, 1991, now abandoned, and divisional application Ser. No. 07/958,596, filed Oct. 8, 1992, now U.S. Pat. No. 5,306,566 issued Apr. 26, 1994 claiming priority from Dutch application Ser. No. 9002022 of Sep. 14, 1990, the contents of which are here incorporated by reference. Particularly described is a laminate composed of a core layer of 0.8 mm thick ABS which is provided on each side with a 0.2 mm thick aluminium skin layer.
When bending this laminate the problem occurs that the laminate behaves more or less as an integral aluminium sheet with a thickness of 1.2 mm. This means that, when making a bend with a very small external radius of curvature, for instance, about 1.5 mm, the metal skin layer at the outside of the bend must be stretched beyond its elastic limit. The result of this is that this outside metal skin layer tends to crack. Heating the laminate, as described in the prior art, does not always prevent this and in some cases it can lead to permanent and undesired change in the physical properties of the laminate.
Other prior art does not provide a method which solves this problem, JP-A-53144975 (abstract) refers to a processing method in which a laminate with thermoplastic core, metal sheets on both faces of the core and a thermosetting resin layer on one metal sheet, is locally heated to above a temperature at which the core layer softens.
In U.S. Pat. No. 4,583,935 a similar method of bending a thermoplastic laminate (without metal skin) is discussed in which localized heating to cause softening of the thermoplastic material is applied only to part of the laminate to be bent. The rest of the laminate remains in a hardened state throughout the bending process. The laminate is heated to a temperature at which ply slippage can occur.